Add `!important` helper fn for dynamic values

[paintparty]

Need an !important helper that aligns with the most common user intention:

(def my-color :red)

(sx [:color (!important my-color)])
;; =>
;; {:class ["_g86523"]
;;  :style {:--my-color my-color}

;; The following css written to disk
;; ._g86523 {
;;   color: var(--my-color)!important;
;; }